1. Field of the Invention
The present invention relates to an apparatus which has an electronic control device to control the assisting force produced by a power-steering system, according to the conditions of a traveling automobile, for varying the ratio of the assisting force to the steering force supplied by the driver.
2. Description of the Prior Art
Usually, a power-steering system of this kind utilizes the velocity of the automobile to detect the conditions of the moving automobile. The assisting force is controlled according to the result of the detection in such a way that the assisting force is increased or decreased, depending on whether the velocity lies in a low-speed range or a high-speed range, respectively. In a known power-steering system as disclosed in Japanese Patent Laid-Open No. 59,574/1984, one characteristic curve is selected from several previously determined characteristic curves according to the road condition, i.e., whether the automobile goes down streets or mountain roads. Each of the characteristic curves determines the relation between the vehicle speed and the rotational frequency of the oil pump. This known system can vary the ratio of the assisting force to the steering effort supplied by the driver, depending on the vehicle speed and also on the road conditions. This ratio will be hereinafter referred to as the "power assistance ratio."
However, the running conditions of the automobile varies not only depending on the aforementioned vehicle speed and road conditions but also depending on the driver's conditions, i.e., the temper and character of the driver. That is, the condition of the automobile changes, according to whether the driver drives boldly or moderately. Therefore, it has not been necessarily possible for the prior art techniques to provide appropriate power assistance ratio according to the driver's conditions.
In order to solve this problem, the present inventors and others proposed a new technique in U.S. patent application Ser. No. 946,050 in 1986. According to this proposed technique, a first driving condition index K.sub..theta. corresponding to the road conditions and a second driving condition index K.sub.v corresponding to the driver's conditions are calculated. Then, the following arithmetic operation is performed. EQU I=I.sub..theta. +I.sub.v
where I.sub..theta. is an electric current applied according to the first index K.sub..theta., and I.sub.v is an electric current applied according to the second index K.sub.v. The sum control current I is applied to a solenoid valve which varies the power assistance ratio. The value of the second term I.sub.v of the formula varies only according to the second condition index K.sub.v. Therefore, the characteristic curve of the control current I is obtained simply by shifting the characteristic curve of the applied current I.sub..theta. upwardly or downwardly according to the second index K.sub.v. For this reason, the control current I cannot vary sufficiently freely. Hence, the power assistance ratio changes merely within a limited range.